CN114641349A

CN114641349A - Method for coating a substrate with a drop on demand printer

Info

Publication number: CN114641349A
Application number: CN202080079039.0A
Authority: CN
Inventors: D·阿克滕; A-C·比拉德荣格; F·舒斯特; T·黑贝施特赖特; J·蒂拉克
Original assignee: Covestro Intellectual Property GmbH and Co KG
Current assignee: Covestro Intellectual Property GmbH and Co KG
Priority date: 2019-11-14
Filing date: 2020-11-10
Publication date: 2022-06-17
Also published as: WO2021094271A1; US20230001691A1; EP4058204A1

Abstract

The 2K paint formulation was printed with a valve jet printer.

Description

Method for coating a substrate with a drop on demand printer

The invention relates to a method for coating a substrate, comprising the step of applying a coating composition drop-wise onto the substrate from a print head, wherein the print head has a closable opening which, according to the instructions of a control unit, allows or prevents the discharge of droplets of the coating composition.

Industrial paints are generally characterized by a combination of protective and decorative functions. Industrial typical paints in application areas such as the transportation, furniture and flooring industry may have a paint layer thickness of 0.01 to 0.2mm, wherein often a plurality of functional plies are combined. Typically, these plies are: a primer as an adhesion promoter for the substrate, a basecoat as a color-imparting layer and a topcoat or clearcoat as a primary protective layer.

There is a need for printing techniques for the effective application of industrial typical paint formulations in the transportation-, furniture-, and flooring industries onto industrial substrates commonly used there.

EP 3257590 a1 discloses a device for maskless painting of aircraft surfaces, comprising a multi-axis robot with at least one applicator for paint, wherein the applicator is mounted to apply paint by means of a technique for spraying paint. The coating can be, for example, an external primer, a base coat, a top coat, a decorative coat, a clear coat, a functional coat, or a protective coat. The technique may be an inkjet technique.

WO 2019/030267 a1 discloses a method for producing A3-dimensional object comprising at least a first and a second object part, wherein the first object part is produced directly on the second object part by means of A3D printing method, the second object part not being produced by the 3D printing method, and wherein the first object part produced by means of the 3D printing method comprises or consists of a polyurethane resin.

WO 2016/046134 a1 discloses a high viscosity valve jet method in which liquid is ejected through an opening of a valve jet printhead. The opening is defined by certain geometrical parameters and the jetting viscosity of the liquid is between 20mPa s and 3000 mPa s.

WO 2019/109040 a1 describes a system for applying a coating to a substrate using a high transfer efficiency applicator. The system includes a high transfer efficiency applicator defining a nozzle. The coating comprises a carrier agent and a binder. The coating has a viscosity of about 0.002 Pa.s to about 0.2 Pa.s, about 838kg/m³To about 1557kg/m³A density of about 0.015N/m to about 0.05N/m, a surface tension of about 0.0005 seconds to about 0.02 seconds.The high transfer efficiency applicator is configured such that it discharges the coating through a nozzle opening onto the substrate to form a layer of paint. At least 80% of the droplets of coating material discharged from the high transfer efficiency applicator are in contact with the substrate.

US 7,927,669B 2 relates to a method of applying a coating, such as in particular a paint, to a surface with an apparatus comprising a metering head having at least one nozzle controllable by a control signal. The method comprises the following steps: -moving a substrate having a surface to be coated along the surface relative to a metering head and/or moving the metering head relative to the surface to be coated of the substrate, and-applying a fluid coating material onto the surface through the nozzle in accordance with at least one computer-generated control signal.

It is an object of the present invention to provide a digital printing process by which coating compositions-other than inks-can be treated.

This object is achieved by a method according to claim 1. Advantageous embodiments are given in the dependent claims. They may be combined in any combination, unless the context clearly dictates otherwise.

In a method for coating a substrate, comprising the step of applying a coating composition drop-wise onto the substrate from a nozzle having a closable opening which, according to the instructions of a control unit, allows or prevents the discharge of a droplet of the coating composition.

It is provided that the coating composition is applied more than once over less than 30% of the area of the substrate, that a pressure of from ≥ 1.2bar to ≤ 3.5bar acts on the coating composition when the nozzle opening is closed, and that the nozzle opening has a diameter of from ≥ 80 μm to ≤ 250 μm.

Further provided is a coating composition comprising a reactive resin, a hardener for the resin and a solvent, the resin and the hardener being present together in a proportion of ≥ 20% by weight, based on the total weight of the coating composition, in the coating composition, and the coating composition having a viscosity at 20 ℃ of ≥ 20mPa · s to ≤ 80mPa · s, measured according to ENISO3219/A3 at a shear rate of 1000/s.

Individual droplets of a solvent-based two-component paint formulation (2K paint formulation) can be applied to a substrate by the method of the present invention, wherein the droplets merge into a continuous layer on the substrate. Then, after evaporation of the solvent, a continuous coating is obtained on the substrate. It has surprisingly been found that coatings meeting the quality requirements can be obtained efficiently and in an economical manner within the parameters provided by the present invention. Standard formulations for lacquers are already available for this purpose. After application, the coating may be cured by conventional methods.

The closable opening of the nozzle means that the coating method is a variant of the so-called drop-on-demand printing method, in particular a coating method by means of a valve-jet printer. The opening of the nozzle can be opened or closed, for example, by means of a magnetically actuated plunger. The frequency at which the opening of the nozzle is opened and closed may preferably be 800Hz to 4000 Hz.

Of course, a plurality of such nozzles may be used. They may be arranged together on a movable print head. The control unit is usually arranged in the apparatus for the method, such as a valve jet printer, and may comprise, for example, a Raster Image Processor (RIP).

The process according to the invention is suitably carried out as a so-called single pass process. This means that the coating composition is provided more than once for as little substrate area as possible. According to the present invention, it is provided that the coating composition is applied more than once on less than 30% of the area of the substrate. Preferably, less than 20% of the area is involved, particularly preferably less than 10% of the area is involved. Ideally, each surface portion of the substrate to be printed is printed only once with the coating composition, apart from technically unavoidable overlaps and inaccuracies.

The method according to the invention differs from the spraying method for applying paint mainly in that larger droplets are ejected. Due to the method, the droplets are precisely positioned. Furthermore, the droplets evaporate significantly less solvent on the short flight path between the nozzle and the substrate. The solvent still present in the formulation can then ensure that the viscosity of the formulation on the substrate is reduced in such a way that a continuous film can be formed from the individual droplets.

With regard to the machine parameters, the inventors have found that a pressure of ≥ 1.2bar to ≤ 3.5bar should be applied to the coating composition when the opening of the nozzle is closed. This can be considered as the "operating pressure" of the valve-jet printer and is easier to determine than the pressure in the case of open nozzles during material discharge. Preferred pressures are from ≥ 1.2bar to ≤ 3 bar, particularly preferably from ≥ 1.4 bar to ≤ 2.5 bar.

The inventors have also found that the diameter of the nozzle opening should be ≥ 80 μm to ≤ 250 μm. The preferred diameter is from ≥ 100 μm to ≤ 200 μm, particularly preferably from ≥ 120 μm to ≤ 150 μm.

The type of the substrate is not particularly limited. The substrate may be planar or curved. The method according to the invention also makes it possible to treat relatively large substrates, for example having an area of more than 1m²Greater than 10m²Or more than 100m²The substrate of (1). Suitable materials may be aluminium, steel or polymers. The substrate may be part of a finished component, for example a machine housing, or may be present as a semi-finished product such as a strip, web or sheet.

The coating composition comprises a reactive resin, a hardener for the resin and a solvent and can therefore be considered as a 2K composition. The composition is only formulated prior to application, since there is only a limited processing time due to the reaction of the resin with the hardener. The typical reaction of the resin with the hardener is a crosslinking reaction, forming a covalent bond. Of course, the composition may also comprise other components such as reactive diluents or additives. Thus, solids-containing coating compositions can also be treated. The d90 value of the particle size distribution of these particles is preferably ≦ 5 μm. The coating composition can be designed as a base coat, a color coat or a top coat.

The proportion of resin and hardener totaling ≧ 20 wt.% further distinguishes the composition from an inkjet ink containing a binder. In such inks, the proportion of binder is significantly lower. Preferably, the total proportion of resin and hardener is ≥ 30 wt.%, more preferably ≥ 40 wt.%, based on the total weight of the coating composition.

The viscosity provided according to the present invention also distinguishes the composition from an inkjet ink containing a binder; in the latter, it is significantly lower. The preferred viscosity is > 25 mPas < 70 mPas, particularly preferably > 30 mPas < 65 mPas.

The process can be carried out such that the desired wet lacquer layer thickness is produced in a material yield of >90%, preferably >95% and very particularly preferably 98%, based on the amount of lacquer used, since no overspray and no significant evaporation of solvents or components on the way to the substrate occur.

The process can also be carried out such that the desired dry lacquer layer thickness is 1/1.5, preferably 1/1.7, very particularly preferably 1/2, of the wet lacquer layer thickness.

The method can also be carried out in such a way that the solvent-borne paints to be applied lose less than 20%, preferably less than 10%, particularly preferably less than 5%, of their solvent on the way to the substrate, so that the inevitable solvent evaporation process can be carried out on the substrate in a controlled manner, whereby the environmental pollution is less and an improved surface quality is obtained compared to conventional spray application processes.

The lacquer layer obtained by the process according to the invention can have a Dry Film Thickness (DFT) of 0.015 to 0.080mm, preferably 0.02 to 0.06mm and very particularly preferably 0.025 to 0.05mm after a single application (single pass printing).

According to one embodiment, the following parameters are chosen such that the characteristic number K, calculated according to the following formula, is ≧ 0.4 to ≦ 4:

wherein:

r: positional resolution of the application of the coating composition, expressed in dots per 2.54cm

p: the pressure acting on the coating composition when the opening of the nozzle is closed is given in bar

d: the diameter of the closable opening of the nozzle, given in μm

S: the proportion of non-volatile constituents of the coating composition is given in% by weight, based on the total weight of the coating composition, according to ISO 3251, 120 min, 100 DEG C

Eta: the viscosity at 20 ℃ measured at a shear rate of 1000/s according to EN ISO3219/A3, expressed in mPas.

This empirically derived formula is derived by the inventors from statistical analysis of experimental results and allows optimal setting of the remaining parameters in the case where a part of the parameters is known. For example, with known machine parameters (resolution, pressure, nozzle diameter given in dpi) and a solids content given by the formulation of the lacquer, the viscosity can be brought into the target range by adding solvents. For the calculation of the feature number K, the physical units are ignored. It depends only on the values of the variables involved.

According to another embodiment, the coating composition is applied drop-wise onto the substrate from a plurality of nozzles, and each nozzle has a closable opening that is independent of the other closable openings of the other nozzles, which allows or prevents the discharge of droplets of the coating composition according to the instructions of the control unit.

According to another embodiment, the coating composition is applied with a positional resolution of ≥ 30 dots/2.54 cm and ≤ 150 dots/2.54 cm. This is common data in the printing industry for resolution in dots per inch (dpi). The preferred resolution is from 40. gtoreq./2.54 cm to 130. gtoreq., particularly preferably from 50. gtoreq./2.54 cm to 120. gtoreq./2.54 cm.

According to another embodiment, the solvent in the coating composition is selected from: water, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, n-octane, isooctane, mineral spirits, xylene, solvent naphtha, propanol, n-butanol, isobutanol, butanediol, butyl diglycol, ethylene glycol, diethylene glycol, butyl acetate, ethyl acetate, 2-butoxyethyl acetate, 1-methoxy-2-propyl acetate, butanone, acetone, 2-heptanone, 2, 4-pentanedione, 2-pentanone, ethyl 3-ethoxypropionate, 1,2, 4-trimethylbenzene, 4-methylpentan-2-one, or a mixture of at least two of the foregoing solvents.

According to another embodiment of the preceding claims, the coating composition has a pot life of ≥ 30 minutes and ≤ 480 minutes, where pot life is defined as the time until the viscosity doubles at 23 ℃ as determined according to DIN EN ISO 3219/a. The pot life is preferably from 60 minutes or more to 240 minutes or less.

According to another embodiment, the resin is an epoxy resin and the hardener is a polymerization catalyst, a primary amine, a cyclic anhydride, a polyphenol, a thiol, or a mixture of at least two of the foregoing compounds.

The epoxy resin and the epoxy hardener form an epoxy resin adhesive as a reaction mixture, which hardens by an polyaddition reaction. Upon curing, the low viscosity or low molecular weight monomeric and oligomeric components of the binder form a high molecular weight three-dimensional network by a crosslinking reaction. The network nodes are created by the reaction of the functional groups of the resin and hardener. Suitable epoxy resins are in particular those based on glycidyl ethers, glycidyl esters, glycidyl amines, cycloaliphatic epoxides and glycidyl isocyanurates.

Examples of hardeners are 1, 3-diaminobenzene, diethylenetriamine, 4' -methylenebis (cyclohexylamine) and hexahydrophthalic anhydride.

According to another embodiment, the resin is a polyol, a polyamine, an aminoalcohol or a mixture of at least two of the above compounds and the hardener is a blocked or unblocked polyisocyanate.

Suitable polyols are the polyether polyols, polyester polyols, polycarbonate polyols, polyesteramide polyols, polyamide polyols, epoxy polyols and their mixtures with CO known from polyurethane chemistry₂And a polyacrylate polyol.

Examples of polyamines are 3-amino-1-methylaminopropane, 3-amino-1-ethylaminopropane, 3-amino-1-cyclohexylaminopropane or 3-amino-1-methylaminobutane. Examples of amino alcohols are N-aminoethylethanolamine, ethanolamine, 3-aminopropanol, neopentanolamine or diethanolamine.

Examples of polyisocyanates are polyisocyanates or polyisocyanate mixtures having exclusively aliphatically and/or cycloaliphatically bonded isocyanate groups, an (average) NCO functionality of from 2.0 to 5.0 and a viscosity at 23 ℃ of from 10 to 2000 mPas. Suitable polyisocyanates are in particular those based on isophorone diisocyanate, hexamethylene diisocyanate, bis- (4-isocyanatocyclohexyl) methane and ω, ω' -diisocyanato-1, 3-dimethylcyclohexane (H6 XDI).

The diisocyanate mentioned may be used as such or a derivative of the diisocyanate may be used. Suitable derivatives are polyisocyanates containing biuret, isocyanurate, uretdione, urethane, iminooxadiazinedione, oxadiazinetrione, carbodiimide, acylurea or allophanate groups. Particular preference is given to monomer-depleted lacquer polyisocyanates having these structural elements, selected from isophorone diisocyanate (IPDI), Hexamethylene Diisocyanate (HDI), 1, 4-diisocyanatocyclohexane or bis- (4-isocyanatocyclohexyl) -methane.

The polyisocyanate component may optionally be hydrophilically modified. Water-soluble or water-dispersible polyisocyanates can be obtained, for example, by modification with carboxylate, sulfonate and/or polyethylene oxide groups and/or polyethylene oxide/polypropylene oxide groups.

Suitable blocking agents for the polyisocyanates are, for example, monoalcohols such as oximes, for example acetone oxime, methyl ethyl ketoxime, cyclohexanone oxime, lactams such as epsilon-caprolactam, phenols, amines such as diisopropylamine or dibutylamine, dimethylpyrazoles or triazoles and also dimethyl malonate, diethyl malonate or dibutyl malonate.

Particular preference is given to coating compositions comprising isocyanurate of 1,6-HDI ("HDI trimer") and polyacrylate polyol. Butyl Acetate (BA), 1-methoxy-2-propyl acetate (MPA), solvent naphtha and mixtures thereof may be used to adjust the viscosity. Silicone additives may be added for affecting surface activity.

According to another embodiment, the NCO index of the coating composition is from ≥ 0.8 to ≤ 1.5, preferably from ≥ 0.9 to ≤ 1.3, particularly preferably from ≥ 1 to ≤ 1.2.

The invention is explained in more detail by means of the following examples, without being restricted thereto.

Method

Dry film thickness (dry film thickness, DFT):

eddy current methods (DIN EN ISO 2360) are applied for measurements on non-magnetic substrates, such as aluminum. It is based on the variation of the magnetic field of an electromagnet caused by eddy currents in a conductive substrate. This magnetic field variation depends on the dry layer thickness of the coating.

For example, layer thickness gauges with integrated or hand-held exchangeable probes are available from the manufacturers Helmut Fischer, Erichsen, BYK-Gardner, Elcometer or TQC.

For determining the dry layer thickness, 9 individual measurements were carried out at different points on the plane to be examined and an average value was formed therefrom.

Visual evaluation: the surface quality was visually assessed on a scale of-5 to + 5: -5 is too thick and wrinkled, +5 is striped or unprinted. Values from-1 to 1 are considered good paint qualities.

Viscosity:

the viscosity is determined in accordance with DIN EN ISO3219/A3 and is carried out with an MCR301 rheometer from Anton Paar. Anton Paar measuring cup: cylinder geometry, measuring cup diameter 28.92mm, cylinder diameter 26.66mm, annular gap length 40mm, measuring temperature 20 ℃, shear rate from 1 to 15001/s in 55x5 seconds =275 seconds and from 15001/s to 1 in 275 seconds.

Material

And (2) component A:

acrylate containing polyols Setalux D A665 (BA/X) from Allnex Resins Germany GmbH or Allnex Netherlands BV.

BYK 331 (10% methoxypropyl acetate) of BYK-Chemie is an organosilicone surface additive for solvent-containing, solvent-free and aqueous paints and printing inks.

And (B) component:

polyisocyanate: HDI trimer DESMODUR ULTRA N3390 BA/SN (NCO functionality > 3) was used, the NCO content was 19.6% by weight (according to ISO 11909), the nonvolatile proportion was about 90% (according to ISO 3251, 120 min, 100 ℃ C.), and the result was from Covestro AG. The viscosity was about 550 mPas at 23 ℃ (DIN EN ISO 3219/A3).

Butyl acetate or 1-methoxy-2-propyl acetate (MPA)/Solvent Naphtha 100 (1:1) was used as Solvent.

An aluminum plate was used as the substrate. It was previously cleaned with ethyl acetate.

Unless otherwise indicated, the starting materials were used without further purification/cleaning or pretreatment.

Formulations

To produce component A, the acrylate-containing polyol or the mixture of two polyols is diluted with BA or MPA/SN and the surface additive is homogeneously mixed. The hardener component was used with the polyisocyanate, NCO: OH ratio = 1.0. The solids content of the mixture was about 50% by weight, optionally further diluted. At a solids content of 50%, the application viscosity (ISO cup 5mm DIN EN ISO 2431) at 23 ℃ is approximately 30 seconds.

Formulations	A	B
				[g]	[g]
Component A
			SETALUX D A 665 BA/X	49.06	49.06
BYK 331	0.50	0.50
			Acetic acid butyl ester	32.04	43.70

			Component B
DESMODUR ULTRA N 3390 BA/SN	18.40	18.40

Sum of	100.00	111.66

Printing parameters

Experiments were performed with a desktop printer model ChromoJET TT version 2.0 from Zimmer Austria. The pre-mixed paint formulation was filled into a 300 mL pressure tank 10 minutes after mixing. An operating pressure of 1 to 3.5bar is set by the pressure regulator. First, the system is rinsed multiple times with the paint formulation and then printed on the cleaned substrate. The print area was 130 x 70 mm. The carriage speed was 0.6 m/s. A print head with 9 nozzle groups with nozzle diameters of 100, 120 or 150 μm was used. After printing was completed, the system was flushed with ethyl acetate.

After a cooling time of 10 minutes at room temperature, the applied lacquer was cured in an oven at 140 ℃ for 25 minutes.

The results marked are according to the invention.

Numbering	1A	1B	2B*
				Description of the invention	Varnish for automobile	Diluted automotive varnish	Diluted automotive varnish
Formulations	A	B	B
				Viscosity [ mPas]At a temperature of 23 ℃ at 1000/s	120	33	33
Solids [% ]]	50	44	44
				Printer parameters
Nozzle diameter [ mu m [ ]]	100	100	100
				Pressure [ bar ]]	3	3	1.5
Resolution x/y [ dpi ]]	76	76	50
				DFT	n.a.	94	28
Visual assessment	5	-3	1
				Number of features K	0.36	15.04	2.36

The following examples are hypothetical examples. These formulations were not printed. Example No. 4 is expected to produce a striped surface, and example nos. 5 and 6 produce satisfactory printed images.

Numbering	3B	4B*	5B*
				Description of the preferred embodiment	Varnish for automobile	Diluted automotive varnish	Diluted automotive varnish
Formulations	B	B	B
				Viscosity [ mPas]At a temperature of 23 ℃ at 1000/s	33	33	33
Solids [% ]]	44	44	44
				Printer parameters
Nozzle diameter [ mu m [ ]]	100	150	120
				Pressure [ bar ]]	1	1.5	1.5
Resolution x/y [ dpi ]]	25	50	76
				DFT	n.a.	n.a.	n.a.
Visual assessment	Stripe	Good effect	Good effect
				Number of features K	0.10	2.59	3.83

The "feature number K" listed in the table is calculated according to the formula already explained above:

the characteristic numbers resulting from the limit values defined according to the invention result in parameter ranges which surprisingly produce the desired surface quality and print quality/thickness on valve inkjet printers for the printed industrial lacquer quality.

Claims

1. A method of coating a substrate comprising the step of applying a coating composition drop-wise from a nozzle onto the substrate,

wherein the nozzle has a closable opening which, upon command of the control unit, allows or prevents the discharge of droplets of the coating composition,

it is characterized in that the preparation method is characterized in that,

applying the coating composition more than once over less than 30% of the area of the substrate,

a pressure of from not less than 1.2bar to not more than 3.5bar is applied to the coating composition when the opening of the nozzle is closed,

the opening of the nozzle has a diameter of more than or equal to 80 mu m and less than or equal to 250 mu m,

the coating composition comprises a resin, a hardener for the resin, and a solvent,

in the coating composition, the reactive resin and the hardener are present together in a proportion of ≥ 20 wt.% based on the total weight of the coating composition, and

the viscosity of the coating composition at 20 ℃ is from not less than 20 mPa.s to not more than 80 mPa.s, measured at a shear rate of 1000/s according to EN ISO 3219/A3.

2. The method according to claim 1, wherein the following parameters are chosen such that the characteristic number K, calculated according to the following formula, is ≥ 0.4 to ≤ 4:

wherein:

d: the diameter of the closable opening of the nozzle, given in μm

S: the proportion of non-volatile constituents of the coating composition, given in% by weight, based on the total weight of the coating composition, is in accordance with ISO 3251, 120 min, 100 DEG C

η: the viscosity at 20 ℃ measured at a shear rate of 1000/s according to EN ISO3219/A3, expressed in mPas.

3. The method of claim 1 or 2, wherein the coating composition is applied drop-wise onto the substrate from a plurality of nozzles and

wherein each nozzle has a closable opening that allows or prevents the discharge of a droplet of the coating composition according to the instructions of the control unit independently of the other closable openings of the other nozzles.

4. The method of any preceding claim, wherein the coating composition is applied with a positional resolution of ≥ 30 dots/2.54 cm to ≤ 150 dots/2.54 cm.

5. The method according to any one of the preceding claims, wherein in the coating composition, the solvent is selected from the group consisting of: water, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, n-octane, isooctane, mineral spirits, xylene, solvent naphtha, propanol, n-butanol, isobutanol, butanediol, butyl diglycol, ethylene glycol, diethylene glycol, butyl acetate, ethyl acetate, 2-butoxyethyl acetate, 1-methoxy-2-propyl acetate, butanone, acetone, 2-heptanone, 2, 4-pentanedione, 2-pentanone, ethyl 3-ethoxypropionate, 1,2, 4-trimethylbenzene, 4-methylpentan-2-one, or a mixture of at least two of the foregoing solvents.

6. The method according to any one of the preceding claims, wherein the coating composition has a pot life of from ≥ 30 minutes to ≤ 480 minutes, wherein pot life is defined as the time until the viscosity doubles at 23 ℃ as determined according to DIN EN ISO 3219/a.

7. The method of any one of claims 1 to 6, wherein the resin is an epoxy resin and the hardener is a polymerization catalyst, a primary amine, a cyclic anhydride, a polyphenol, a thiol, or a mixture of at least two of the foregoing compounds.

8. The method of any of claims 1-6, wherein the resin is a polyol, a polyamine, an amino alcohol, or a mixture of at least two of the foregoing compounds, and the hardener is a blocked or unblocked polyisocyanate.

9. The method of claim 8, wherein the NCO index in the coating composition is 0.8 or more and 1.5 or less.